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In this chapter you will: o o o o o o Learn about client/server architecture Study server-side scripting Create ASP .NET applications Learn about object collections Study ASP .NET’s core objects Create a guest book

CHAPTER

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our Web browser is a client in the client/server environment of the Web. Up to this point, this book has focused on client-side JavaScript development in Web browsers. For you to develop a full complement of Web development skills, you also need to understand the server side of the Web— in particular, how server-side scripting fits into Web development. In this chapter, you will study server-side scripting with ASP.NET. But before getting into server-side scripting, it’s helpful to set the stage by reviewing the fundamentals of client/server architecture.

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CLIENT/SERVER ARCHITECTURE
There are many definitions of the term’s client and server. In traditional client/server architecture, the server is usually some sort of database from which a client requests information. A server fulfills a request for information by managing the request or serving the requested information to the client—hence the term, client/server. A system consisting of a client and a server is known as a two-tier system. One of the primary roles of the client, or front end, in a two-tier system is the presentation of an interface to the user.The user interface gathers information from the user, submits it to a server, or back end, then receives, formats, and presents the results returned from the server. The main responsibility of a server is usually data storage and management. Heavy processing, such as calculations, on client/server systems usually takes place on the server. As desktop computers become increasingly powerful, however, many client/server systems place at least some of the processing responsibilities on the client. In a typical client/server system, a client computer may contain a front end that is used for requesting information from a database on a server. The server locates records that meet the client request, performs some sort of processing, such as calculations on the data, then returns the information to the client. The client computer may also perform some processing such as building the queries that are sent to the server or formatting and presenting the returned data. Figure 11-1 illustrates the design of a two-tier client/server system.

Client request Server response

Client

Server

Figure 11-1

The design of a two-tier client/server system

The Web is built on a two-tier client/server system, in which a Web browser (the client) requests documents from a Web server.The Web browser is the client user interface.You can think of the Web server as a database of Web pages. Once a Web server returns the requested document, the Web browser (as the client user interface) is responsible for formatting and presenting the document to the user. Once you start adding databases and other types of applications to a Web server, the client/server system evolves into what is known as a three-tier client architecture. A three-tier, or multitier, client/server system consists of three distinct pieces: the client tier, the processing tier, and the data storage tier.The client tier, or user interface tier, is still the Web browser. However, the database portion of the two-tier client/server system

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is split into a processing tier and the data storage tier.The processing tier, or middle tier, is sometimes called the processing bridge. It handles the interaction between the Web browser client and the data storage tier. Essentially, the client tier makes a request of a database on a Web server.The processing tier performs any necessary processing or calculations based on the request from the client tier, and then reads information from or writes information to the data storage tier. The processing tier also handles the return of any information to the client tier. Note that the processing tier is not the place processing takes place. The Web browser (client tier) still renders Web page documents (which requires processing), and the database or application in the data storage tier may also perform some processing. Client-side JavaScript exists at the client tier, while serverside JavaScript exists at the processing tier. Figure 11-2 illustrates the design of a threetier client/server system.
Two-tier client/server architecture is a physical arrangement in which the client and server are two separate computers. Three-tier client/server architecture is more conceptual than physical, because the processing tier and data storage tier can be located on the same server. Multitier client/server architecture is also referred to as n-tier architecture.

Client tier

Processing tier

Data storage tier

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Handles user interface display (the Web browser) and submits requests to the processing tier

Handles interaction between the Web browser client and the data storage tier

Stores data in a database and returns requests presented by the processing tier

Can be the same computer

Figure 11-2

The design of a three-tier client/server system

An important question in the design of any client/server system is deciding how much processing to place on the client. In the context of Web site development with JavaScript, you must decide whether to use client-side or server-side JavaScript. This decision may sound confusing, because client-side JavaScript and server-side JavaScript share much of the same core language. However, whether to use client-side or server-side JavaScript is important consideration because the choice you make can greatly affect the performance

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of your program. A general rule of thumb is to allow the client to handle the user interface processing and light processing, such as data validation, but have the server perform intensive calculations and data storage.This division of labor is especially important when dealing with the Web. Unlike with clients on a private network, it’s not possible to know in advance the computing capabilities of each client on the Web.You cannot assume that each client (browser) that accesses your client/server application (Web site) has the necessary power to perform the processing required by the application. For this reason, intensive processing should be placed on the server. Because servers are usually much more powerful than client computers, your first instinct may be to let the server handle all processing and only use the client to display a user interface. Although you do not want to overwhelm clients with processing they cannot handle, it is important to perform as much processing as possible on the client for several reasons: s Distributing processing among multiple clients creates applications that are more powerful because the processing power is not limited to the capabilities of a single computer. Client computers become more powerful every day, and advanced capabilities such as JavaScript and DHTML are now available in local Web browsers. Thus, it makes sense to use a Web application to harness some of this power and capability. (A Web application is a program that executes on a server but that clients access through a Web page loaded in a browser.) Local processing on client computers minimizes transfer times across the Internet and creates faster applications. If a client had to wait for all processing to be performed on the server, a Web application could be painfully slow over a busy Internet connection. Performing processing on client computers lightens the processing load on the server. If all processing in a three-tier client/server system is on the server, the server for a popular Web site could become overwhelmed trying to process requests from numerous clients.
The term distributed application is used to describe multiple computers sharing the computing responsibility for a single application.

s

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SERVER-SIDE SCRIPTING
Chapter 5 touched on three-tier client/server processing in relation to server-side scripting languages that execute on a Web server. Some of the more popular server-side scripting languages that are used to process form data include Common Gateway Interface (CGI), Java Server Pages (JSP), and Active Server Pages (ASP). Server-side scripting languages exist on the processing tier and have the ability to handle communication between the client tier and the data storage tier. At the processing tier, a scripting language usually prepares and processes the data in some way before submitting it to the data storage tier. Additionally, server-side JavaScript can interact closely with client-side JavaScript, because they share the same basic programming features. For these reasons, many developers prefer server-side scripting languages for handling the processing tier of a three-tier client/server system. Server-side scripting languages are not without their drawbacks. One of their biggest disadvantages is that server-side scripting languages are proprietary and vendor specific. There is no server-side scripting language standard similar to ECMAScript. Although server-side scripting languages are proprietary and vendor specific, most implementations use much of the same core language syntax that you have learned for client-side JavaScript. You can write server-side scripts using many of the skills you have already learned in client-side JavaScript, regardless of the server-side scripting language you use. In this chapter, you will use ASP.NET, which is a server-side scripting language created by Microsoft. ASP.NET is the latest version of Active Server Pages and is part of the Microsoft Visual Studio .NET line of software development products. You can create ASP.NET using one of the following Windows platforms: s s s

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Windows 2000 (Professional, Server, and Advanced Server) Windows XP Professional Windows Server 2003 family for both client and server applications

To use ASP.NET, you must also install Internet Information Services (IIS), which are used for creating, configuring, and managing Web sites. IIS is available with each of the preceding platforms. See your Windows documentation for information on installing and configuring IIS.
The examples and figures in this chapter were created using Windows XP Professional.

The Microsoft Visual Studio .NET line of software development products includes a platform called the .NET Framework, which is designed for creating, deploying, and running Web applications and services. In order to create ASP.NET applications, you must install the .NET Framework. ASP.NET is automatically installed as part of the

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Windows Server 2003 family. For Windows 2000 and Windows XP Professional, ASP.NET and the .NET Framework are installed as part of Visual Studio .NET or through the Microsoft .NET Framework SDK. If you do not have Visual Studio .NET, you can download the Microsoft .NET Framework SDK free from http://msdn.microsoft.com/ downloads/default.aspx.
Because the goal of this book is to teach JavaScript, this chapter provides only a brief overview of ASP.NET. The installation and configuration requirements for ASP.NET, the .NET Framework, or IIS are beyond the scope of this book. For installation and configuration procedures, along with additional information on working with ASP.NET, see the Microsoft ASP.NET page at http://msdn.microsoft.com/asp.net/. If you are in a classroom setting, ask your instructor for specific instructions on how to work with ASP.NET at your institution.

The ASP.NET techniques you study in this chapter are identical in both ASP and ASP.NET. Although the chapter focuses on ASP.NET, you can also perform the exercises in this chapter with ASP if you do not have access to ASP.NET. Active Server Pages are supported by Internet Information Server 3.0 and higher running on Windows NT Server 4.0 and Windows 2000, Peer Web Services running on Windows NT Workstation 4.0, and Personal Web Server running on Windows 95/98. Older versions of ASP (without the .NET extension) use a file extension of .asp. You save ASP.NET documents in a folder that you designate as a virtual directory in IIS. Once you create a virtual directory in IIS, you can open any files it contains in a Web browser using the following syntax: http://computer_name/directory/file. For example, if your computer is named personal_computer and you create a virtual directory named interests, you can open a document named index.html by typing the following statement in a Web browser’s address bar: http://personal_computer/interests/ index.html. Alternatively, you can replace the computer name with localhost, as follows: http://localhost/interests/index.html. The term localhost is an alias for the name of a computer.
You can change the name of your computer using the System Properties dialog box in the Control Panel.

In this chapter, you will create a Web site for a company named WebAdventure; the Web site will use ASP.NET to store the number of hits it receives, and it will include an area where visitors can sign a guest book. The program you create will not be a complete, robust Web site, because it includes only a hit counter and a database. However, it will serve to demonstrate how to create a basic program with ASP.NET. Next, you will create a virtual directory in IIS for the WebAdventure Web site.The following instructions are for Windows 2000, Windows XP, and Windows Server 2003. If

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you are running Peer Web Services or Personal Web Server, refer to your Windows documentation for information on creating a virtual directory. To create a virtual directory in IIS for the WebAdventure Web site: 1. Open the Control Panel from the Start menu. 2. If you are using Windows XP, click the Switch to Classic View link to display the Control Panel icons. 3. Use the Administrative Tools icon to open the Administrative Tools window. 4. Select Internet Information Services. The Internet Information Services window opens. 5. Click the plus sign next to the icon that represents your computer, and then click the plus sign next to the Web Sites folder, if necessary. 6. Click the plus sign next to the Default Web Site icon that appears beneath the computer name icon. Figure 11-3 shows the Internet Information Services window on a computer named DONGOSSELIN.

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Figure 11-3

Internet Information Services window

7. Verify that the Default Web Site icon is selected, click Action on the menu bar, point to New, and then click Virtual Directory. The Virtual Directory Creation Wizard appears. 8. In the introductory dialog box of the Virtual Directory Creation Wizard, click the Next button to display the Virtual Directory Alias dialog box. 9. In the Virtual Directory Alias dialog box, type webadventure, and then click the Next button. The Web Site Content Directory dialog box appears.

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10. Type the path where you store your JavaScript projects. For example, if your JavaScript projects are stored in a folder named JavaScript_Projects on your C: drive, you would type C:\JavaScript_Projects. Click the Next button when you are finished. The Access Permissions dialog box appears. 11. Leave the options in the Access Permissions dialog box set to their default values, and click the Next button to display the final Virtual Direction Wizard Creation dialog box. 12. Click Finish to create the virtual directory. 13. After you create the webadventure virtual directory, close Internet Information Services and the Administrative Tools window.

CREATING ASP.NET APPLICATIONS
ASP.NET scripts are created as text files, the same as XHTML documents, and can contain both client-side JavaScript code and server-side JavaScript code. Documents for ASP.NET applications have the .aspx file extension. ASP applications files have an .asp extension. When a client requests an ASP.NET document, the Web server executes any server-side scripting code before serving the document to the client. Once the client Web browser receives the document, it executes the client-side JavaScript. Be aware that the first time a client requests an ASP.NET document from a server, the server compiles the document, regardless of whether it contains server-side JavaScript code.The term compiling refers to the processing and assembly of programming code into an executable format. The difference between interpreting and compiling is that, while interpreted programs (such as JavaScript programs) are processed and assembled into an executable format each time they execute, compiled programs only need to be recompiled when their code changes. You do not need to compile a server-side script yourself because ASP.NET automatically recognizes any changes to an ASP.NET application and recompiles the application the next time a client requests it. Thus, installing an ASP.NET application is as simple as placing the documents that make up the application in one of the server’s directories.Additionally, an ASP.NET application starts automatically the first time a Web server receives a request for one of the application’s pages. The term ASP.NET application refers to a collection of related ASP.NET documents that exist in the same root directory.
Because the compilation process requires extra processing time, you should not use the .aspx or .asp extensions with XHTML documents that do not contain server-side JavaScript code, because XHTML documents do not need to be compiled.

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Code Declaration Blocks
You define ASP.NET global variables and functions within code declaration blocks, which you create with elements, the same as client-side JavaScript.You use three attributes in a code declaration block: runat=”server”, language, and src. The runat=”server” attribute is required in order to identify the script section as an ASP.NET script. The language attribute identifies the scripting language used within the code declaration block. You can use three scripting languages with ASP.NET: VB (Visual Basic), JScript (JavaScript), and C# (a .NET Framework programming language that is part of Visual Studio). Although the language attribute is optional, ASP.NET uses Visual Basic by default, so you must include the language attribute in order to use JavaScript with your ASP.NET applications.The src attribute is the same attribute that is used in client-side JavaScript to identify a source document containing scripting code. A major difference between using the element for ASP.NET instead of client-side JavaScript is that all code, with the exception of global variables, must be contained within a function. For example, the following code contains an example of a code declaration block that includes a single Write() method of the Response object within a function named sayHello(). functionƒsayHello()ƒ{ Response.Write(“HelloƒWorld”); }

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By comparison, the following code declaration script would generate a compile error because the statement is not contained within a function: Response.Write(“HelloƒWorld”);

Because XHTML does not recognize the ASP.NET run attribute and because the language attribute is deprecated, you cannot validate the ASP.NET documents you create. However, even though you cannot validate your ASP.NET documents, you should still write well-formed XHTML elements and attributes, especially for any elements and attributes that will be returned as a response to the user. Notice that the Write() method in the preceding examples is appended to the Response object in the same manner that the write() method in client-side

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JavaScript is appended to the Document object. Also notice that the Response object and Write() method are written with their first letter in uppercase. In ASP.NET, objects and methods are usually written with uppercase first letters.You will learn about the Response object later in this section.The Write() method works the same as the client-side write() method, except that instead of designating text to be output to the browser, it designates information to be returned to the client. You cannot execute a function contained within a code declaration block. Instead, you must execute a function from a code render block, which you will study next. Next, you will start creating a document that will be used as the start page for the WebAdventure Web site. To start creating a document that will be used as the start page for the WebAdventure Web site: 1. Open your text editor and create a new document. 2. Type the declaration, element, header information, and the element. Use the strict DTD and “WebAdventure” as the content of the element.Your document should appear as follows: WebAdventure

3. Add the following ASP.NET code declaration block to the document head. The script section uses the Date object to generate the date, which it sends to the client using a Response.Write() statement. functionƒtimeStamp()ƒ{ ƒƒƒƒƒƒƒvarƒnowƒ=ƒnewƒDate(); ƒƒƒƒƒvarƒdayƒ=ƒnow.getDay(); ƒƒƒƒƒvarƒdateƒ=ƒnow.getDate(); ƒƒƒƒƒvarƒyearƒ=ƒnow.getFullYear(); ƒƒƒƒƒvarƒmonthƒ=ƒnow.getMonth(); ƒƒƒƒƒvarƒhoursƒ=ƒnow.getHours(); ƒƒƒƒƒvarƒminutesƒ=ƒnow.getMinutes(); ƒƒƒƒƒvarƒsecondsƒ=ƒnow.getSeconds(); ƒƒƒƒƒvarƒdaysƒ=ƒnewƒArray(); ƒƒƒƒƒdays[0]ƒ=ƒ“Sunday”;ƒdays[1]ƒ=ƒ“Monday”;ƒ ƒƒƒƒƒdays[2]ƒ=ƒ“Tuesday”;ƒdays[3]ƒ=ƒ“Wednesday”;ƒ ƒƒƒƒƒdays[4]ƒ=ƒ“Thursday”;ƒdays[5]ƒ=ƒ“Friday”;

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ƒƒƒƒƒ days[6]=”Saturday”; ƒƒƒƒƒvarƒdisplayƒ=ƒdays[day]ƒ+ƒ“ƒ“ƒ+ƒmonthƒ+ƒ“/”ƒ+ƒdateƒ+ƒ“/” ƒƒƒƒƒƒƒƒƒƒƒ+ƒyearƒ+ƒ“ƒ“+ƒhoursƒ+ƒ“:”ƒ+ƒminutesƒ+ƒ“:”ƒ+ƒseconds; ƒƒƒƒƒResponse.Write(“Theƒcurrentƒdateƒandƒtimeƒareƒ“) ƒƒƒƒƒResponse.Write(display); ƒƒƒƒƒResponse.Write(“”); }

4. Add to the document body the following heading element, a form that contains a text field for the user’s first name, and a Submit button that calls the main document, named HomePage.aspx. The Submit button submits the value in the text field to the HomePage.aspx document.
PleaseƒenterƒyourƒfirstƒnameƒandƒclickƒContinue toƒproceedƒtoƒourƒhomeƒpage. FirstƒName:ƒ

5. Save the document as StartPage.aspx in your Chapter folder for Chapter 11.

Code Render Blocks
ASP.NET also uses the script delimiters to designate server-side JavaScript code. A delimiter is a character or a sequence of characters used to mark the beginning and end of a code segment.You include within the script delimiters any commands that are valid for the scripting language you are using.

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The preceding statement creates a code render block, which executes ASP.NET code within a Web page document.You can declare the language attribute in a code render block, but you must use an ASP processing directive, which provides a Web server with information on how to process the scripts in an ASP document. ASP processing directives are created using the delimiters.You can only declare one processing directive on a page, so it is usually declared at the top of a document or above the first code render block. For example, the following statement includes an ASP processing directive, which declares that the code render block is written in JScript:

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One of the primary reasons to use a code render block is to include ASP.NET inside an XHTML element. As an example, consider the Session object, which is used in ASP.NET to create custom client properties. Assume that you have created a custom property, named email, in the Session object, and you want to use that property in a link that uses the XHTML mailto: protocol.To return the custom email property of the ASP.NET Session object and use it in the link, you use syntax similar to the following: Clickƒhereƒtoƒsendƒanƒe-mail
You cannot declare a function within a code render block.

Notice in the preceding code that you refer to the email property of the Session object as an argument of the Contents object. Properties in ASP.NET are stored in data structures known as collections instead of as properties of objects. The Contents object referenced in the preceding code is actually called the Contents collection.You will learn about collections at the end of this section. An important difference between code declaration blocks and code render blocks is that any variables you define within a code render block are only available to other code render blocks on the page.You can include multiple code declaration blocks and code render blocks in a document. However, variables that you want to be globally available to all other ASP.NET code sections must be declared within a code declaration block.
Although you can include multiple code declaration blocks and code render blocks in a document, you can only use one scripting language: VB (Visual Basic), JScript (JavaScript), or C#. In other words, the value you assign to the language attributes in multiple script sections must match.

The output directive sends the result of an expression to a user’s Web browser (the client). The syntax for the output directive is . For example, if your script includes a variable named salesTotal that has been assigned a value of $24.95, then the output directive sends the value $24.95 to the client Web browser.
The output directive is equivalent to the Response object Write() method.

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The following code shows how to use the output directive in a code render block.The example combines the output directive with the Response object Write() method, and includes the language attribute to designate JavaScript as the default scripting language. The expression in the output directive calculates firstNum and secondNum, then sends the text “The result of 10 minus 2 is 8” to the user’s Web browser. Figure 11-4 shows the output. CodeƒRenderƒBlock

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Figure 11-4

ASP.NET code render block example

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Next, you add a processing directive and a code render block to the StartPage.aspx document. To add a processing directive and a code render block to the StartPage.aspx document: 1. Return to the StartPage.aspx document in your text editor. 2. Add the following processing directive to the top of the document. The processing directive declares JScript as the script language.

3. Now add a code render block after the closing tag in the document head. The code render block executes the timeStamp() function you added in the last exercise.

4. Save the StartPage.aspx document, and then open it in your Web browser by typing the following URL in the address box: http://localhost/ webadventure/Chapter.11/ Chapter/StartPage.aspx.You cannot open StartPage.aspx as a local file using the Open command from your Web browser’s File menu.You must type the full URL to your server. 5. After the document opens, view the source document. (In Internet Explorer, select Source from the View menu.) In the text editor that opens, notice that the document does not contain the ASP.NET statements you added. Instead, only the output from the Response.Write() statements is available to the client. Figure 11-5 shows how the document appears in a Web browser. 6. Close your Web browser window.

Figure 11-5

StartPage.aspx

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Mixing XHTML and ASP.NET
The ASP.NET code within code declaration blocks and code render blocks execute on a Web server before the page is sent to a user. If users were to view the source document after they received the ASP document, they would not see any elements, script delimiters, or ASP.NET code they contain. Instead, the client shows only the results returned by the code. For example, the following code shows an ASP document containing client-side JavaScript as well as ASP.NET code render blocks and code declaration blocks. Notice that the function declared within the code declaration block is executed in the last code render block. MixedƒExample functionƒcodeBlockText()ƒ{ ƒƒƒƒƒResponse.Write(“CodeƒDeclarationƒBlock”);ƒ ƒƒƒƒƒResponse.Write(“ThisƒlineƒisƒgeneratedƒbyƒanƒASP.NET codeƒdeclarationƒblock.”); }

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The following code shows how the preceding document appears once a client receives it. Notice that the client-side JavaScript code is the same, but the ASP.NET code has been converted to elements and text. Figure 11-6 shows how the text and elements appear in a Web browser. MixedƒExample CodeƒRenderƒBlockThisƒlineƒisƒgeneratedƒbyƒanƒASP.NET codeƒrenderƒblock. CodeƒDeclarationƒBlockThisƒlineƒisƒgeneratedƒbyƒan ASP.NETƒcodeƒdeclarationƒblock.

Figure 11-6

Result of a document with multiple script sections displayed in a Web browser

You can see in the preceding example that XHTML elements and text can be interspersed with script delimiters. However, you can go even further and include XHTML elements and text as part of an ASP.NET decision-making structure, such as an if...else statement. For example, in the following code, because each if or else...if statement is enclosed by script delimiters, it is unnecessary to use the Write() method of the Response object to output the correct greeting. Only one greeting is returned, based on the results of the if...else statement. ƒƒƒƒƒBuenosƒDias ƒƒƒƒƒGutenƒTag

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ƒƒƒƒƒBuonƒGiorno ƒƒƒƒƒBonjour ƒƒƒƒƒIƒdon’tƒspeakƒyourƒlanguage!

You could also write the code within a single pair of script delimiters, as the following code illustrates. However, the greetings are not treated as XHTML elements or text, because they are enclosed by the script delimiters. For this reason, each greeting must include a Response.Write() statement to be returned to the client.

The following code shows another example of interspersing ASP.NET with XHTML elements and text to generate a customized Web page.The example shows an XHTML document that could be generated in response to a client request for information. Assume that the document from which the client request originates includes a form field named clientName. The example uses the clientName property of the Content collection of the Request object and uses an output directive to intersperse the property throughout the text to be returned to the client, creating a customized response. The example also assumes that the company’s e-mail address is stored in the email property of the Content collection of the Application object. Figure 11-7 shows the output in a Web browser, assuming that “Don” is the value of the clientName property. MixedƒExample Dearƒ: ThankƒyouƒforƒyourƒinterestƒinƒourƒWebƒdevelopmentƒservices. Asƒyouƒknow,ƒitƒisƒdifficultƒtoƒkeepƒupƒwithƒrapidlyƒchanging technology.ƒWithƒWebAdventure’sƒhelp,ƒyouƒcanƒbeƒconfident thatƒyourƒWebƒsiteƒwillƒbeƒonƒtheƒcuttingƒedge.ƒRemember,

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,ƒWebAdventureƒisƒhereƒfor you.ƒForƒmoreƒinformation,ƒclickƒhereƒtoƒsendƒusƒan e-mail.

Figure 11-7

ASP.NET interspersed with XHTML in a Web browser

OBJECT COLLECTIONS
Before learning about ASP.NET core objects, you need to understand collections. Collections are data structures similar to arrays that store variables in ASP.NET core objects. The syntax for assigning a variable to a collection is object.collection (“variable”)ƒ=ƒvalue;. You then refer to the variable in ASP.NET code by using the syntax object.collection(“variable”);. For instance, the Application object and Session object both contain separate Contents collections, which store custom variables. The following code creates a name variable in the Session object Contents collection, assigns it a value of “Don Gosselin”, and then sends the value to the client browser, using a Response.Write() statement: Collections are commonly used in Visual Basic programming. Much of ASP.NET non-JavaScript syntax comes from Visual Basic programming, because the ASP.NET default scripting language is VBScript, which is derived from Visual Basic.

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If a variable name you assign to a collection is unique throughout all of the collections of an object, then you can eliminate the collection name when referencing the object in code. For example, if the Session object contains only one name variable in all of its collections, then you can refer to the name variable in code by using a statement such as Response.Write(Session(“name”));. It is usually safer, however, to include the collection name in order to eliminate any uncertainty as to what collection contains a specific variable.Additionally, eliminating the collection name could cause bugs in your program if you later add the same variable name to another collection in the same object. Therefore, the code in this chapter includes collection names when referencing variables. Each variable in a collection is numbered, similar to the way element numbers are assigned to arrays.You can refer to collection variables by using numbers instead of variable names. For example, the following code creates three variables, firstName, lastName, and email, in the Contents collection of the Session object, then returns the values to the client, using the collection number of each variable.

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Be aware that the number assigned to a variable in a collection can change if you remove items from the collection. You should not assume that a number always represents the same variable in a collection.
You can remove items from a collection using the Remove() and RemoveAll() methods of the Contents collection. Both the Application object and Session object contain Contents collections.

ASP.NET collections support a count property, which returns the number of variables in a collection.You can use the count property in a looping statement to cycle through the variables in a collection. For example, the following code uses a for loop to return the variables in the Contents collection to the client.The for loop conditional evaluation compares the curVariable variable to the count property. While curVariable is less than the count property, the for loop continues iterating through the collection.

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